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What caused the first mass extinction on Earth? – Aayush, 10, New Jersey

Dear Aayush,

It’s sad when living things go extinct. That means they’re gone forever. I think about extinct unicorns all. the. time.

My friend Jodi Rosso told me that a mass extinction is when a huge number of species die out all at once. She teaches earth science at Washington State University.

“There are periods when a whole lot of animals and plants die off in a short time,” Rosso said. “When we say a short time, we mean less than 2 million years. From a human perspective, that's forever. But in geologic time, that's short.”

Groups of living things are always expanding or shrinking their numbers. Scientists think there are as many as 10 million species on Earth right now. Between 100 and 10,000 of them go extinct every year.

A mass extinction is when big percentage of all species die off.

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How did our planet get to be the one with breathable air? – Jacob, 11, Idaho

Dear Jacob,

When I wake up from a cat nap, I stretch and take a deep breath. It feels good to fill my lungs with oxygen.

But that wasn’t always possible on Earth.

I talked about this with my friend Sean Long. He’s a geologist at Washington State University.

“The cool thing is the answer has to do with life,” Long said. “Early life forms on Earth gave us all the oxygen. They were single-celled bacteria.”

Our planet is about 4.5 billion years old. There’s been life on Earth for 3.5 billion years. The first life forms were made of just one cell. They were bacteria and their cousins called archaea.

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